Fine carbonaceous substances such as a carbon nanotube (CNT), carbon nanofiber, fullerene, and carbon black are superior in electric conductivity and thermal conductivity in addition to having lightweight, high-strength, and high-rigidity characteristics, so that it is expected to considerably improve physical characteristics of a metal material by uniformly dispersing them in a metal.
Conventionally, as a method to highly strengthen a metal represented by magnesium, it has been thought that a carbon nano-material is dispersed in a metal, for example.
For example, Japanese Unexamined Patent Publication No. 2007-154246 (patent document 1) discloses a technique to produce a composite powder by making a carbon nanomaterial adhere to a surface of a metal powder particle such as magnesium and aluminum through a mechanical alloying process and then pack and consolidate the composite powder.
Japanese Unexamined Patent Publication No. 2005-200723 (patent document 2) discloses a technique to disperse carbon nanofibers in a gel-like dispersion liquid provided by adding borax to a polyvinylalcohol aqueous solution, add a metal powder thereto and knead them, and obtain a carbon nanofiber-metal based material.
However, since the nanosized to micro-sized fine carbonaceous substance is likely to aggregate due to Van der Waals' force between carbon atoms, the fine carbonaceous substance cannot be uniformly dispersed onto the metal powder particle by the above method, so that it is difficult to dramatically improve characteristics of the metal material in a final stage.
One inventor of this application, FUGETSU disclosed in WO2005/110594A1 (patent document 3) that carbon nanotubes are successfully monodispersed in a solution, using amphiphilicity of a surfactant having hydrophilicity and hydrophobicity.
The other inventor of this application, KONDOH thought that the characteristics of the metal material can be dramatically improved by uniformly dispersing the carbon nanotubes in the metal material, using the above technique by FUGETSU. Thus, KONDOH and FUGETSU started a challenge, as collaborative study to produce a CNT-uniformly-dispersed alloy by making the carbon nanotubes uniformly adhere to a surface of a metal powder particle using the above technique by FUGETSU, and uniformly dispersing the carbon nanotubes in a metal or an alloy by a solid-phase reaction between composite metal powder particles.
At first, it was thought that a highly-strengthened metal material could be obtained in final stage just by soaking metal powder particles in the CNT dispersed solution produced by FUGETSU and making the CNTs adhere to a metal powder surface. However, the strength was not exactly improved.
As one of its reasons, it is thought that the CNTs adhere to the metal powder too much, and the metal powder particle surface is completely covered with the CNTs, which prevents the solid-phase reaction between the metal powder particles.
As another reason, it is thought that as the CNTs adhere to the metal powder surface via a component of a dispersion liquid (hereinafter referred to as the “binder”), the strength of the final metal material is lowered because of the presence of this binder component when the metal powder is heated to be solidified. When the binder is not completely removed, metallurgical combination (sintering) of the metal powder particles is prevented. Furthermore, when the metal powder particles are solidified with the binder remaining on the powder surface, the binder is thermally decomposed and generates a gas and the gas exists as a blister in the solidified metal material, which lowers the strength of the material.
In order to improve the characteristics of the final metal material, it is considered preferable that the CNTs adhere to the metal powder surface in a monodispersed state with the metal powder surface partially exposed. Alternatively, it is preferable to minimize a residual amount of the binder serving as an adhesive agent to make the CNTs adhere to the metal powder particle surface. In addition, it is necessary to find a condition to thermally decompose the binder.